SPECIAL SENSES
v The special senses of
hearing,sight,smell and taste all have specialized sensory receptors (nerve
endings) outside the brain.
v These are found in the ears,eyes,nose
and mouth.
v The ear is also involved in the
maintenance of balance.
v In the brain the incoming nerve
impulses undergo complex processes of integration and coordination that result
in perception of sensory information and a variety of responses inside and
outside the body.
HEARING AND THE
EAR
1.INTRODUCTION: The ear is the organ hearing.
2.SITUATION:The structures that form the ear are
encased within the petrous portion of the temporal bone.
3.STRUCTURE:
The ear is divided into 3 parts.
1.
Outer ear
2.
Middle ear
3.
Inner ear .
OUTER EAR :
It consists
of the auricle (pinna) and the external acoustic meatus.
THE AURICLE (PINNA):
v The auricle is the expanded portion
protecting from the side of the head.
v It is composed of fibroelastic
cartilage covered with skin .
v It is deeply grooved and ridged and
the most prominent outer ridge is the
helix.
v The lobule (earlobe) is the soft
pliable part at the lower extremity, composed of fibrous and adipose tissue
richly supplied with blood.
EXTERNAL ACOUSTIC MEATUS (AUDITORY
CANAL)
v This is slightly ‘S’ Shaped tube about 2.5 cm long extending from
the auricle to the tympanic membrane(eardrum).
v The lateral third is cartilaginous
and the remainder is a canal in the temporal bone.
v The meatus is lined with skin
containing hairs continous with that of the auricle.
v There are numerous sebaceous and ceruminous
glands in the skin of the lateral third.
v Ceruminous glands are modified sweat
glands that secrete cerumen (wax).
CERUMEN (WAX):
Ø It is a sticky material containing
lysozyme and immunoglobulins.
Ø e.g foreign materials….dust,insects
and microbes are prevented from the reaching the tympanic membrane by wax,hairs
and the curvature of the meatus.
Ø Movements of the temporomandibular
joint during chewing and speaking ‘massage’, the cartilaginous meatus , moving
the wax towards the exterior.
THE TYMPANIC MEMBRANE (EAR DRUM).
Ø It separates the external acoustic
meatus from the middle ear.
Ø It is oval-shaped with the slightly
broader edge upwards and is formed by three types of tissue.
Ø The outer covering of hairless skin ,
the middle layer of fibrous tissue and the inner lining of mucus membrane
continous with that of the middle ear.
MIDDLE EAR (TYMPANIC CAVITY)
Ø This is an irregular –shaped air
filled cavity within the petrous portion of the temporal bone.
Ø Its contents and the air sacs which
open out of it are lined with either simple squamous (or) cuboidal epithelium.
Ø The lateral wall of the middle ear is
formed by the tympanic membrane .
Ø The roof and floor are formed by the
temporal bone with openings leading to the “mastoid antrum” through which are passess to the air cells within
the mastoid process.
The medial
wall is a thin layer of temporal bone, it has 2 openings.
1.
Oval window
2.
Round window
The oval window is occluded by part of a small bone called
the “stapes” and the round window by a fine sheet of fibrous tissue.
Air reaches the cavity through the pharyngo tympanic(auditory
or eustachian) tube. Which extends from the nasopharynx.
It is 4 cm long and lined with ciliated columnar epithelium .
The tympanic membrane is maintained by the pharyngotympanic tube
and enables the membrane to when sound waves strike it.
AUDITORY OSSICLES:
These are 3 very small bones which are present in middle ear
of tympanic membrane to the oval window.
They form a series of movable joints with each other. They
are
1.THE MALLEUS
2.THE INCUS
3.THE STAPES
1.THE MALLEUS:-
ü It is the lateral hammer –shaped
bone.
ü The handle is in contact with the
tympanic membrane.
ü The head forms a movable joint with
the incus.
2.THE INCUS
ü It is the middle anvil-shaped bone.
ü Its body articulates with the
malleus,the long process with the stapes.
ü It is stabilized by the short
process, fixed by fibrous tissue to the posterior wall of the tympanic cavity.
3.THE STAPES
ü It is the medial stirrup – shaped
bone.
ü its head articulates with the incus
and its foot plate fits into the oval window.
ü The three ossicles are held in
portion by fine ligaments.
INNER EAR
The inner ear also called it has labyrinth.
It contains the organs of hearing and balance.
It is described into 2 parts.
1.Bony labyrinth
2. Membranous
labyrinth.
1.BONY LABYRINTH :
§ It’s a cavity with in the temporal
bone lined with periosteum.
§ It is a layer and encloses the
membranous labyrinth of the same shape tube.
§ There is a layer between the bony and
membranous labyrinth is called “perilymph”or watery fluid.
§ Within the membranous labyrinth there
is a similarly watery fluid ‘’Endolymph’.
Bony labyrinth consists of
1.
Vestibule
2.
Cochlea
3.
3 semicircular canals.
1.Vestibule:
It is the extended part nearest
the middle ear and contains the oval and round window in lateral wall.
The cochlea:
It’s a snails shape , it has a broad base where it continous with th
vestibule and narrow apex.
3.semi circular canals:
These are three tubes arranged so that one is situated in each of the three
planes of space .
They continous with the vestibules.
MEMBRANOUS LABYRINTH:
This contains endolymph and lies with in bony counter part. It has
1. The vestibule which contains the
utricle and saccule.
2. The cochlea .
3. Three
semicircular canals.
THE COCHLEA:
It contains 3 sections
1.
The scala vestibule
2.
The scala media or cochlear duct
3.
The scala tympani.
The bony
cochlea has 2 compartments contains ‘perilymph.
·
i’e
the scala vestibule
·
scala
tympani.
§ The cochlea duct is a part of the
membranous labyrinth and it is in
triangular shape.
§ The basilar membrane (or) base of the
triangle there are supporting cells and specialized cochlear hair cells containing auditory
receptors.
§ These cells form the spinal organ (of
corti), the sensory organ that responds to vibration by initiating nerve
impulses that are then perceived as hearing by the brain.
§ The auditory receptors are dendisites
of efferent nerves that combine forming the cochlear (Auditory) part of the
“Vestibulocochlear nerve” (8th cranial nerve).
§ 8th cranial nerve passes
through a foramen in the temporal bone to reach the hearing area in the
temporal lobe of the cerebrum.
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